Having no idea what this "Gemfile" is that I should be editig I heading to Google. After some hair pulling I opted to rather to write my own quick watch script (which is a bit quicker as well as I'm only syncing the folder where the change happened).

First run:

vagrant ssh-config >>> ~/.ssh/conf

If you have not already, then head over to ~/.ssh/conf with a text editor like nano or vi[m].

Edit the entry named Host default that Vagrant created to anything that you would like, in my case I'll keep it as Host default.

Now in your project file create the following file replaceing <vagrant-host-here> with your chosen name for the vagrant host (in my case default):